The present invention relates to radar controlled weapons systems and, more particularly, to a method and apparatus for generating real-time high resolution Synthetic Aperture Radar (SAR) imagery from an airborne platform of a translating ship under the influence of roll, pitch, and yaw motions characteristic of sea state conditions.
A highly resolved ship image on an airborne display permits targeting to a particular part of a ship for the purpose of standoff command guidance weapon delivery. As such, this invention relates to airborne SAR systems used for generating real-time high resolution imagery of a ship target under the influence of sea state conditions and for accurately measuring and tracking the range and azimuth angle of a designated resolution cell within the aforesaid displayed target area so as to enable the accurate delivery of an air-to-ground missile or glide bomb from the SAR bearing aircraft to the ship target. The manner of weapon guidance depends upon reducing to zero the relative range and azimuth angle between weapon and designated target resolution cell.
Existing airborne synthetic aperture radar systems have the capability for producing a high resolution image of stationary ground targets by virtue of the rotational motion of the SAR with respect to the target field. High resolution in both range and cross-range (azimuth) directions are achieved with short-pulse radar techniques for range resolution, and by digital signal doppler processing for azimuth resolution, so as to produce a two-dimensional image. The application of such known SAR processing techniques to a ship at sea, however, can readily produce distorted and improperly focused ship imagery, precluding ship recognition, because of ship rotational motions brought about by sea state conditions.
Systems for ameliorating some of the adverse effects brought about by ship rotational motions are described in the commonly assigned copending U.S. patent applications of Sol Boles, entitled "Range/Azimuth Ship Imaging For Ordnance Control", Ser. No. 389,367, and entitled "Range/Doppler Ship Imaging For Ordnance Control", Ser. No. 389,368, filed concurrently herewith, the disclosures of which are incorporated herein by reference. The first invention provides an undistorted two-dimensional image of the ship from a direct plot of range versus interferometrically determined azimuth angle of all essential scatterers comprising the ship. The second invention, on the other hand, provides an improvement in image definition, as well as performance to greater ranges, by displaying range versus doppler, after removal of the isodop type distortions inherent in the formed range/doppler image.
Notwithstanding the advantages achieved in conjunction with each of the above-identified inventions, they do have certain operational limitations and drawbacks in certain applications. For example, each of the aforementioned inventions is subject to the limitations on image resolution brought about by the distributed elevation angles of the essential scatterers comprising the ship target.
The invention disclosed herein is related to the inventions described in the aforementioned applications of Boles, but by a unique processing implementation, is capable of removing any restrictions due to ship scatterer elevation on the formation of a scaled high resolution image. This advantage is brought about by utilizing elevation angle, as well as azimuth angle, interferometric techniques in conjunction with SAR signal processing techniques.
It is accordingly an object of the present invention to provide a method and apparatus for generating high resolution synthetic aperture radar displayed imagery from an airborne platform of a ship under the influence of sea state conditions.
It is another object of the present invention to provide a method and apparatus for eliminating the distortions inherent in displayed ship imagery brought about by the distributed elevation angles of the essential scatterers comprising the ship target.
It is still another object of the present invention to provide focussed high resolution imagery of a ship target so as to permit the cursoring and tracking of a particular target resolution cell for the purpose of carrying out standoff command guidance weapon delivery to said target.
It is a still further object of the present invention to provide a method and apparatus for providing scaling of the cross-range (doppler) dimension of Inverse SAR Profile Imagery.